On Thu, Mar 21, 2024 at 01:43:45PM -0700, Anatoliy Klymenko wrote:
> diff --git a/drivers/gpu/drm/drm_crtc_helper.c b/drivers/gpu/drm/drm_crtc_helper.c
> index 2dafc39a27cb..f2e12a3c4e5f 100644
> --- a/drivers/gpu/drm/drm_crtc_helper.c
> +++ b/drivers/gpu/drm/drm_crtc_helper.c
> @@ -1055,3 +1055,39 @@ int drm_helper_force_disable_all(struct drm_device *dev)
>  	return ret;
>  }
>  EXPORT_SYMBOL(drm_helper_force_disable_all);
> +
> +/**
> + * drm_helper_crtc_select_output_bus_format - Select output media bus format
> + * @crtc: The CRTC to query
> + * @crtc_state: The new CRTC state
> + * @supported_fmts: List of media bus format options to pick from
> + * @num_supported_fmts: Number of media bus formats in @supported_fmts list
> + *
> + * Encoder drivers may call this helper to give the connected CRTC a chance to
> + * select compatible or preffered media bus format to use over the CRTC encoder
> + * link. Encoders should provide list of supported input MEDIA_BUS_FMT_* for
> + * CRTC to pick from. CRTC driver is expected to select preferred media bus
> + * format from the list and, once enabled, generate the signal accordingly.
> + *
> + * Returns:
> + * Selected preferred media bus format or 0 if CRTC does not support any from
> + * @supported_fmts list.
> + */
> +u32 drm_helper_crtc_select_output_bus_format(struct drm_crtc *crtc,
> +					     struct drm_crtc_state *crtc_state,
> +					     const u32 *supported_fmts,
> +					     unsigned int num_supported_fmts)
> +{
> +	if (!crtc || !supported_fmts || !num_supported_fmts)
> +		return 0;
> +
> +	if (!crtc->helper_private ||
> +	    !crtc->helper_private->select_output_bus_format)
> +		return supported_fmts[0];
> +
> +	return crtc->helper_private->select_output_bus_format(crtc,
> +							crtc_state,
> +							supported_fmts,
> +							num_supported_fmts);
> +}

Again, the output of that selection must be found in the CRTC state,
otherwise CRTCs have no way to know which have been selected.

Maxime